1. Field of the Invention
The present invention relates to a resonant line for use in a resonance circuit such as a voltage controlled oscillator.
2. Description of the Related Art
The circuit of a voltage controlled oscillator is formed on a printed board, for example, as shown in FIG. 5. The collector of an oscillation transistor 11 is connected to the ground in a high frequency manner. A feedback capacitor 1 is provided between the base and emitter. A feedback capacitor 13 is provided between the emitter and ground. A microstrip line 14 serving as a resonant line is provided between the base and ground. The microstrip line 14 is formed by conductive foil or the like on a printed board (not shown). An oscillation frequency is changed by a varactor diode 15 connected in parallel with the microstrip line 14.
Since the microstrip line 14 is used as an inductance device in this case, one end is connected to the base side of the oscillation transistor 11 and the other end is connected to the ground. The length of the microstrip line 14 is set to the oscillation frequency, that is, a value shorter than a quarter of a wavelength at a resonance frequency determined by the feedback capacitors 12 and 13, microstrip line 14, varactor diode 15 and the like.
The oscillation frequency is adjusted by the microstrip line 14. At this time, generally, a part of the microstrip line 14 is trimmed so as to obtain a predetermined oscillation frequency in a state were a predetermined tuning voltage is applied to the varactor diode 15.
It is necessary to make the microstrip line shorter as the oscillation frequency of the voltage controlled oscillator is increased. Even when the microstrip line is trimmed slightly, however, the oscillation frequency largely changes and it is difficult to adjust the frequency to a target frequency.
It is therefore an object of the invention to provide a resonant line by which a frequency is easily adjusted even when it is used at a high oscillation frequency.
In order to achieve the object, a resonant line of the invention comprises a plurality of microstrip lines. The length of each of the microstrip lines is set so that the reactance between one end of the microstrip line and the ground is equivalently inductive at a predetermined frequency and one ends of the microstrip lines are connected to each other.
According to the resonant line of the invention, the length of each of the microstrip lines is set to a value shorter than a quarter of a wavelength at a predetermined frequency and the other end of each of the microstrip lines is connected to the ground.
According to the resonant line of the invention, the length of at least one of the plurality of microstrip lines is set to a value longer than a quarter of a wavelength at a predetermined frequency and shorter than a half of the wavelength, the other end of the microstrip line is opened, the length of the other microstrip line is set to a value shorter than a quarter of the wavelength, and the other end of the other microstrip line is connected to the ground.
According to the resonant line of the invention, the length of each of the microstrip lines is set to a value longer than a quarter of a wavelength at a predetermined frequency and shorter than a half of the wavelength, the other end of the microstrip line is connected to the ground, and a concentrated constant device serving as an inductance device is connected in parallel with the plurality of microstrip lines.
According to the invention, there is also provided a resonant line comprising a plurality of first microstrip lines which have the same length and are connected in parallel with each other and a second microstrip line connected to one end of each of the plurality of microstrip lines, wherein when the length of each of the first microstrip lines is L1, the length of the second microstrip line is L2, and the number of the first microstrip lines is N, L1/N+L2 is set to a value shorter than a quarter of the wavelength at a predetermined frequency.